National Instruments NI-VXI - Manual (Page 89)

Chapter 3 Software Overview
© National Instruments Corporation 3-51 NI-VXI User Manual
VXI Trigger Functions
VXI triggers are a backplane feature that VXI added to the VME
standard. Tight timing and signaling is important between many types
of controllers and/or instruments. In the past, clumsy cables of
specified length had to be connected between controllers and/or
instruments to get the required timing. For many systems, phase
shifting and propagation delays had to be calculated precisely, based on
the instrument connection scheme. This limited the architecture of
many systems.
In VXI however, every VXI board with a P2 connector has access to
eight 10 MHz TTL trigger lines. If the VXI board has a P3 connector, it
has access to six 100 MHz ECL trigger lines. The phase shifting and
propagation delays can be held to a known maximum, based on the
VXIbus specification’s rigid requirement on backplanes. The VXIbus
specification does not currently prescribe an allocation method for TTL
or ECL trigger lines. The application must decide how to allocate
required trigger lines.
The VXIbus specification specifies several trigger protocols that can be
supported, thereby promoting compatibility among the various VXI
devices. The following is a description of the four basic protocols.
• SYNC—SYNC protocol is the most basic protocol. SYNC protocol
is a pulse of a minimum time (30 ns on TTL, 8 ns on ECL) on any
one of the trigger lines.
• ASYNC—ASYNC is a two-device, two-line handshake protocol.
ASYNC uses two consecutive even/odd trigger lines (a
source/acceptor line and an acknowledge line, respectively). The
sourcing device sources a trigger pulse (30 ns TTL, 8 ns ECL
minimum) on the even trigger line (TTL0, TTL2, TTL4, TTL6,
ECL0, ECL2, or ECL4) and waits for the acknowledge pulse on the
next highest odd trigger line (TTL1, TTL3, TTL5, TTL7, ECL1,
ECL3, or ECL5). The acceptor waits for the source pulse on the
even trigger line. Sometime after the source pulse is sensed (no
maximum time is specified), the acceptor sends an acknowledge
pulse back on the next highest odd trigger line to complete the
handshake.
• SEMI-SYNC—SEMI-SYNC is a one-line, open collector,
multiple-device handshake protocol. The sourcing device sources a
trigger pulse (50 ns TTL, 20 ns ECL minimum) on any one of the
trigger lines. The accepting device(s) must begin to assert the same
trigger line upon reception (within 40 ns TTL, 15 ns ECL